The rise in complexity and sophistication of electronically controlled devices and systems has been observed to generate a concomitant need for higher quality and very stable sources of power. This need particularly has been witnessed in connection with computer systems, including modems, printers and copiers.
Directly supplied utility power alone is found to be unacceptable for such equipment as a consequence of line power anomalies now typically encountered. These anomalies are manifested as any of a variety of phenomena. For example, out of specification voltages, referred to as "sags," are represented as a reduction in rms voltage values over a half cycle interval or more. Where such voltage reductions persist within important grids, they are conventionally referred to as "brown outs."
In addition to "sags" as above described, over-voltage excursions referred to as "surges" may be encountered which, in general, are manifested as deviations above nominal rms value lasting for more than half a cycle. These surges generally are encountered in conjunction with load dropping activities.
Subcycle abnormalities also are witnessed in the line power supplies. For example, high voltage, short-term spikes may occur. Such excursions have been observed to be caused, inter alia, by lightening strikes or sub-station or capacitor switching by a utility.
Static noise conditions also may be encountered in the line power supplies. Such noise phenomena will include common mode noise occasioned by the operation of electrical equipment in close proximity to the source being relied upon or through load switching. Further, transverse mode noise also may be encountered appearing line-to-line and having similar causation.
When encountered within a computer environment, the above cataloged anomalies in line power will have a variety of effects. Line noise may result in data error, unprogrammed jumps and software/data file alterations. Momentary under- and over-voltage generally results in automatic computer power down.
Efforts to overcome the anomalies of line power supplies have evolved a variety of power conditioning devices. One such device is the uninterruptible power supply (UPS). A UPS system consists of a battery power supply, an inverter, a number of switches and a control circuit. A number of different types of UPS systems have been devised including on-line, off-line and interactive UPS systems.
The input to a single phase UPS system consists of three conductors, line, neutral and safety ground, which connect to the corresponding three lines of a power utility. The output of a UPS system consists of line, neutral and safety ground conductors which are connected to the load. A control circuit monitors the three line power supply sensing any anomalies in voltage supplied by the utility during the time when the UPS system is in standby mode. In the event of such an anomaly, the control circuit derives a control input which opens the switches which connect the UPS system to the line power supply of the power utility. The UPS system then is disconnected from the line power supply, and the UPS system operates in a backup mode. In backup mode, power then is delivered from the battery power supply through an inverter to the load. Backfeed protection switches are opened and the battery power supply is engaged in such a way as to provide a continuous supply of power to the load.
Because of the widespread growth of technology and increasing globalization, the market for UPS systems is worldwide. In designing UPS systems, manufacturers must be aware of and comply with the regulations regarding such systems in each of the countries in which it plans to market a system. Approximately eighteen countries in Europe have joined together to form the European Committee for Electrotechnical Standardization (CENELEC). Subject to certain conditions, each European Standard promulgated by the CENELEC must be given the status of a national standard without any alteration. A European Standard may be amended by the CENELEC and each exists in the three official versions, English, French and German. Participating countries include Austria, France, Germany, Italy, Spain and the United Kingdom among others.
While the United States is not a member of the CENELEC and thus not subject to its regulations, manufacturers in this country typically comply with the regulations promulgated by the Underwriter's Laboratory. The relevant UL standards governing UPS systems are UL 1778 and UL 1950. Similarly, other countries, such as Australia and the far East, which are not members of the CENELEC have promulgated their own national standards.
While there may be some similarities among some countries as to some requirements, regulation throughout the world lacks uniformity. Lack of uniformity led to the formation of the CENELEC, but the European Standards have only alleviated the problem to a small degree. Among the CENELEC and other nations, the differences among the varying regulations are substantive not merely formalistic. Several differences between the European and American standards are illustrative. In the United States, the National Electric Code (NEC) requires that safety and neutral lines be tied to ground at an entry box or panel before the power supply from the utility enters a building. There is no European Standard which requires such grounding. Another example is the number of conductors required to open when a non-separately derived UPS system operates in backup mode. The United States requires only the singular opening of the line conductor, while in Europe, two switching relays are required to open both the line and neutral conductors. EN 50091-1-1.
In countries where the neutral and safety lines are not required to be tied to ground at the box or panel, as they are in the United States, a problem has been encountered involving "creeping voltage." UPS systems are generally connected to a load, such as a computer, with EMI capacitors which are used to reduce emissions and radio-frequency interference. These capacitors are connected between the line and neutral conductors, between the line and safety conductors and between the neutral and safety conductors. The voltage at the neutral line should be constantly 0 Volts. When the UPS system operates in backup mode, the capacitors effectively act as a voltage divider between the active, neutral and safety conductors, the capacitance between the active conductor and chassie ground and that between neutral and chassie ground being of approximately equal value. Where the neutral conductor is not connected to frame or UPS safety ground, and thus is left floating, a voltage exists at the neutral conductor with respect to ground. This creeping voltage, experienced by the neutral conductor with respect to ground, is approximately half of the total voltage of the utility which for such total of 230 Volts, as in Europe, is 115 Volts. A voltage of that magnitude may cause damage to sensitive computer equipment.
The problem of creeping voltage does not occur in the United States where the neutral conductor is connected to ground at the panel, effectively maintaining zero voltage with respect to frame ground. For other countries which do not require such a connection, the problem of creeping voltage must be addressed. One solution to the problem is a hard tie between the neutral and safety conductors. Such a tie is required in some countries, such as Australia. However, in other countries, predominantly those in Europe, the active and neutral lines of the line power supply may be switched, which occurs as much as fifty percent of the time. When the active and neutral lines are switched, a hard tie connects the active line, instead of the neutral line, to frame ground which may cause a short-circuit to the utility. Therefore, while a hard tie is a solution for UPS systems in some countries, such as Australia, in Europe a different solution is required.
As the above indicates, maintaining a zero voltage at the neutral line may not be a problem at all in one country, a problem with a simple solution in another and a problem with an as yet undiscovered solution in another. Currently, manufacturers of UPS systems have been forced to develop, supply and provide support for a number of different systems which address this voltage problem but remain in compliance with differing regulatory standards and line setups.